1. Field of the Invention
The present invention relates to a differential amplifier circuit having first and second transistors wherein an input voltage is applied between base electrodes thereof and an emitter feedback resistor is inserted between emitter electrodes thereof.
2. Description of the Related Art
FIG. 1 is schematic circuit diagram showing a differential amplifier circuit as a most basic elementary circuit of a bipolar transistor circuit. In the figure, Q1 and Q2 each indicate a transistor. I represents a current source of a current of 2Io. E is an input voltage source of a voltage `v`.
Equation (1) below gives a relation between an input voltage v and an amount of change (an alternating current component) `i` in a collector current Io of the transistors Q1 and Q2 in the circuit, that is, a transfer characteristic thereof. EQU i=Io*exp(v/Vt)/{1+exp(v/VT)} (1)
VT represents a thermal voltage (=kT/q; where k is Boltzmann's constant, T is the absolute temperature, and q is a charge of an electron). The thermal voltage is approximately 26 mV at room temperature.
The circuit has a very narrow range of linearity. In order to improve the linearity, a feedback resistor R is inserted between emitter electrodes and the current source I is divided into current sources I1 and I2 as shown in FIG. 2. A transfer characteristic of the circuit is analytically insoluble. Input voltage v expressed by output current I is given by equation (2) below. EQU v=VT*1n{(Io+i)/(Io-i)}+2*i*RE (2)
where RE is a resistance of feedback resistor R.
Equation (3) below is given by calculating a higher order derivative of v from equation (2) and converting the derivative into a derivative of i, utilizing a differential characteristic of an inverse function thereof. EQU i=Io[(1/2)(v/VE) EQU -(1/24)(VT/VE)(v/VE).sup.3 EQU -(1/480){3-(5*VT/VE)} EQU (VT/VE)(V/VE).sup.5 ] (3)
VE is expressed by equation (4) below. EQU VE=Io*RE+VT (4)
(VE/VT) represents a reduction rate of a gain of feedback resistor R. Equation (4) indicates that a dynamic range widens roughly in proportion to an increase in (VE/VT).
FIG. 3 shows differential gains of input voltage v normalized by 2*VE when (VE/VT)=3, 5 and 10. A degree of linearity required greatly depends on application areas. When (VE/VT)=10, a differential gain changes by as much as 1 percent with a signal amplitude of half of 2*VE.
When a very high degree of linearity is required, however, too much increase in (VE/VT) causes problems. The problems are mainly: a reduction in gain; an increase in signal-to-noise ratio; and an increase in direct current offset voltage.
A typical application where a high degree of linearity and a low noise are required is, for example, a first amplification stage of a radio communication circuit. In such an application a trade-off between a noise characteristic and linearity has been sought or a large current has been required for improving a noise characteristic.